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The third-order nonlinear optical properties (second molecular hyperpolarizabilities, γ), of a series of organoboron compounds containing dimesitylboron moieties have been investigated by third harmonic generation (THG) measurements at 1.907 μm. Symmetric systems with B(mes) 2 groups (mes=2,4,6-Me 3 C 6 H 2 ) at both ends, e.g. (mes) 2 B-Y-B(mes) 2 where Y=(C 6 H 4 ) n (n=1,2) or trans-trans-CH=CH(C 6 H 4 ) n -CH=CH- (n=1,2), were prepared as well as push-pull systems of the form D-Y-B(mes) 2 where D=MeS, Me 2 N, H 2 N, etc., and Y=C 6 H 4 , C 6 H 4 -C=C, C 6 H 4 -CH=CH-, C 6 H 4 -CH=CH-C 6 H 4 . The B(mes) 2 group compares favorably with the more commonly used NO 2 acceptor function for enhancing γ. Values of γ increase significantly with increasing length of π-conjugation for both symmetric and unsymmetric molecules. The MeS group is much more efficient than MeO for enhancing γ. Values as high as 229 x 10 -36 esu are reported for trans-trans-(mes) 2 -B-CH=CH-(C 6 H 4 ) 2 -CH=CH-B(mes) 2 which has a λ max for absorption of 370 nm. Crystal and molecular structures of the bis(dimesitylboryl) benzene and anthracene compounds 1,4-C 6 H 4 -{B(mes) 2 } 2 and 9,10-C 14 H 8 {B(mes) 2 } 2 are reported, as are fluorescence maxima for the symmetric molecules.